Soft-magnetic, Fe- or Co-based, amorphous alloys produced by liquid quenching methods such as a single roll method, etc. are free from magnetocrystalline anisotropy because of no crystal grains, having small magnetic hysteresis loss, low coercivity and excellent soft magnetic properties. Because of these properties, amorphous alloy ribbons are used in magnetic cores for various transformers, choke coils, saturable reactors and magnetic switches, magnetic sensors, etc. Particularly, Fe-based, amorphous alloy ribbons have relatively high saturation magnetic flux densities Bs, low coercivity, and low loss, gathering much attention as energy-saving, soft-magnetic materials. Among the Fe-based, amorphous alloy ribbons, amorphous Fe—Si—B alloy ribbons having excellent thermal stability are widely used in transformer cores (see, for example, JP 2006-45662 A).
Though amorphous Fe—Si—B alloys have low coercivity and small magnetic hysteresis loss, it is known that their eddy current loss (iron loss-hysteresis loss) in a broad sense is larger than a classical eddy current loss determined under the assumption of uniform magnetization by tens of times to about 100 times. The difference between the broad-sense eddy current loss and the classical eddy current loss is called anomalous eddy current loss or excess loss, which is mainly caused by non-uniform magnetization change. Large anomalous eddy current loss in this amorphous alloy is presumably due to the fact that magnetic domains in the amorphous alloy have large width, resulting in a high speed of domain wall displacement, and thus a large speed of the non-uniform magnetization change.
Known as methods for reducing anomalous eddy current loss in amorphous alloy ribbons are a method of mechanically scratching a surface of an amorphous alloy ribbon (JP 62-49964 B), and a laser-scribing method of irradiating a surface of an amorphous alloy ribbon with laser beams to cause local melting and rapid solidification, thereby dividing magnetic domains (JP 3-32886 B, JP 3-32888 B and JP 2-53935 B).
In the method of JP 3-32886 B for dividing magnetic domains, an amorphous alloy ribbon surface is melted locally and instantaneously by the irradiation of laser pulses in a transverse direction, and then rapidly solidified to form substantially circular recesses in lines. Each recess has a diameter of 0.5 mm or less, particularly 200-250 μm when the recesses are formed before annealing, and 50-100 μm when they are formed after annealing. The recesses have an average interval of 1-20 mm. In a diameter range of 50-250 μm, the iron loss decreases as the diameter increases. With respect to the relation between iron loss and ribbon thickness, the thinner the ribbon, the smaller the iron loss, and a thinner ribbon provides a smaller iron loss-reducing effect by the irradiation of laser pulses, 40-50% at the thickness of 60 μm, and about 10-20% at the thickness of 30 μm or less. In Example 1 of JP 3-32886 B, recesses having diameters of about 50-250 μm are formed with 5-mm intervals by a YAG laser on a 65-μm-thick, amorphous alloy ribbon.
Molten alloy splashes are observed around recesses formed by the method of JP 3-32886 B. This appears to be due to the fact that to form recesses with large intervals on a relatively thick amorphous alloy ribbon, deep recesses are formed by a large irradiation energy density of laser beams. It has been found, however, that when deep recesses are formed at such a large irradiation energy density of laser beams that splashes are formed around the recesses, particularly a relatively thin amorphous alloy ribbon would suffer increase in apparent power (exciting VA) and decrease in a space factor despite the decreased iron loss. Increase in the apparent power of the amorphous alloy ribbon results in larger sound noise when used for distribution transformers, etc. The space factor has the same meaning as a lamination factor LF, smaller LF providing larger ribbon-laminated cores. Increase in the apparent power and decrease in the lamination factor have more serious problems on thinner amorphous alloy ribbons, because thinner amorphous alloy ribbons are more influenced by laser-scribed surface conditions than thicker amorphous alloy ribbons.
The method of JP 3-32888 B for dividing magnetic domains comprises the steps of irradiating an amorphous alloy ribbon with laser pulses having a beam diameter of 0.5 mm or less with an energy density of 0.02-1.0 J/mm2 per one pulse in a transverse direction, so that an amorphous alloy ribbon surface is locally and instantaneously melted and rapidly solidified, thereby forming substantially circular recesses at a line density of 10% or more, and annealing the ribbon. This method is an improvement of the method of JP 3-32886 B, optimizing the distribution density of recesses and the timing of annealing to improve iron loss and exciting properties. In Example 1 of JP 3-32888 B, a 65-μm-thick, amorphous alloy ribbon is irradiated with laser pulses having a beam diameter of 0.2 mm and an energy density of about 0.3 J/mm2, which is supplied from a YAG laser, to form lines of recesses at line density of about 70%. However, molten alloy splashes are observed around recesses shown in JP 3-32888 B. This seems to be due to the fact that deep recesses are formed by a large irradiation energy density of laser beams. As a result, the apparent power increases despite the decreased iron loss.
JP 3-32888 B describes an energy density of 0.02-1.0 J/mm2 per one pulse. However, when laser pulses having as low energy as near 0.02 J/mm2 are projected to an amorphous alloy ribbon as thick as 65 μm, the resultant recesses are not fully deep relative to the thickness of the amorphous alloy ribbon, failing to obtain a sufficient iron loss-reducing effect.
The method of JP 2-53935 B is the same as those described in JP 3-32886 B and JP 3-32888 B, in that an amorphous alloy ribbon is irradiated with laser beams in a transverse direction to melt the surface locally. However, the former is different from the latter in that molten portions are crystallized regions. The crystallized regions are formed by the scanning of laser beams, etc., a ratio d/D of their depth d to the thickness D of the amorphous alloy ribbon being 0.1 or more, and the percentage of the crystallized regions being 8% or less by volume based on the entire ribbon. However, because the molten portions are crystallized regions, the iron loss is not sufficiently reduced.